Method for borehole measurement of formation properties

ABSTRACT

The present invention is a method of estimating formation properties by analyzing acoustic waves that are emitted from and received by a bottom hole assembly.

PROPERTIES

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 09/686,735 filed Oct. 10, 2000, now abandoned.

FIELD OF THE INVENTION

[0002] The present invention is a method of estimating a formation'sproperties, more specifically pore pressure, using a bottom holeassembly which has not yet penetrated the formation.

BACKGROUND OF THE INVENTION

[0003] The pore-fluid pressure of a rock formation, which is alsoreferred to as simply the pore pressure, is measured relative to normalpressure at the depth of the formation, in other words relative to thehydrostatic pressure of a column of water at that depth. During thedrilling of a petroleum well, accurate knowledge of formation porepressure is necessary to ensure that formation fluids do not flow intothe wellbore, which can potentially cause well blow-outs. A drillingfluid, usually referred to as drilling mud or simply mud, with desiredweight and Theological properties is maintained in the wellbore as theprimary method for controlling formation fluid flow. A problem with theuse of drilling mud, however, is that if the pressure in the wellboreresulting from the mud's weight is too high, relative to the porepressure, the drilling rate may be decreased unnecessarily. In addition,if the pressure resulting from the mud's weight is excessively highcompared to the pore pressure, that pressure may exceed the formationfraction pressure, potentially causing a loss of mud into the formation,and/or a well control problem. It is preferable therefore if the mudsused in drilling wells result in an optimum range of wellbore pressure,relative to pore pressure, such that wells may be drilled safely butexpediently. This is often difficult, unfortunately, because accuratepre-drill knowledge of pore pressures is not always available,especially in areas with few previously drilled wells or where thegeology is complex.

[0004] More specifically, drilling operations at present generallyattempt to ensure that the wellbore pressure at any given depth is about0.5 pounds per gallon (0.5 ppg) greater than the highest estimated porepressure at that depth. This 0.5 ppg wellbore pressure safety margin isin part required due to industry's present inability to accuratelypredict pore pressures in the various formations through which thedrilling assembly will drill. Reducing the uncertainty in knowing thepore pressure ahead of the bit would lead to significant reductions inthe cost of drilling operations, as a result, for example, of animproved ability to specify casing setting depths and of an increase inthe rate at which wells are drilled. The present invention allowscontinuous estimation of pore pressures of formations ahead of thedrilling assembly, and thereby allows drilling operations to be carriedout with lower average mudweights—in other words with mud weights whichare optimized for the formations to be drilled and thereby do notrequire use of as large a pressure margin as is current practice.

[0005] Data presently used to estimate the pore pressure profile versusdepth at proposed well locations include offset well data, surfaceseismic data, seismic-while-drilling data, and geologic models. Pressuremeasurements from nearby offset wells can provide the most accuratepre-drill pressure information, but for remote locations these data aregenerally not available. Pore pressure estimation from surface seismicdata is based on an empirical relationship between the velocity of soundwaves in the formation and pore pressure, with assumptions made for thenature of the formation, in other words the type of rocks that areexpected to be present (which is also referred to as formationlithology). This relationship is based on a number of differentproperties which are understood in industry. For example, formationvelocity estimation from seismic data using normal moveout analysistechniques is well understood in the art Equally well understood is thefact that formation velocity is a function of both the elastic moduliand the density of the rock, and that formation velocity generallyincreases with depth as rocks become more and more compacted. It is alsounderstood that an increase in pore pressure with depth often coincideswith a decrease in this increasing velocity trend (or even an actualdecrease in velocity with depth) because the higher pore pressure isassociated with less compacted rock. These combined factors allowderivation of empirical velocity-pore pressure relationships for usewith seismic data.

[0006] Pore pressure predictions from seismic data analysis typicallysuffer from large uncertainty however. There are several contributingfactors to this uncertainty, including the inherent uncertainty in thevelocity models, the uncertainties in the variation of lithologycompared with the data used to build the velocity-pore pressureempirical relationships, and the low vertical resolution of the seismicdata. In addition, large and significant pore pressure variation canoccur over vertical intervals of rock much thinner than that whichseismic data can resolve.

[0007] Seismic-while-drilling (SWD) is a method for estimating formationvelocity above and below the drill-bit during the drilling process.Geophones and/or hydrophones placed at the earth's surface around thewell being drilled record the seismic signals produced by the drill-bitas it drills into the formation. Although the drill bit may emitfrequencies across the acoustic band up to or above approximately 20kiloHertz (20 kHz), only the frequencies in the seismic band (which willbe understood to those skilled in the art as less than about 100 Hz.,and more specifically less than about 80 Hz.) propagate to the surface.In addition to the seismic band signals, an acoustic signal from thedrill bit also propagates along the drill string assembly to thesurface. The signal to be used to determine formation velocity isdetected by cross correlating the signal propagating through the earthwith the signal that has propagated along the drill string. See forexample the disclosure of Staron, Arens, and Gros, in U.S. Pat. No.4,718,048 titled “Method of Instantaneous Acoustic Logging Within aWellbore.” That signal is usually at a single frequency, typically about50 Hz, and, using inversion processing, which is analogous to surfaceseismic processing, can be used to estimate the acoustic impedance andvelocity of intervals below the drill bit. Pore pressure is thenestimated using the same velocity-pore pressure empirical relationshipsused with surface seismic data.

[0008] Compared to pore pressure prediction using surface seismicmethods, the main advantages of SWD are that the depth to sub-surfacereflectors is better constrained and vertical resolution is improved.Unfortunately, there are also some important limitations with SWD. Forexample, the resolution of analytic results from SWD data is generallylimited by the relatively low seismic wave frequencies. Second, poor SWDsignals are received with polycrystalline diamond compact (PDC) bits,which are generally the preferred bits for drilling operations wherehigh pore pressure is expected to be encountered. Traditionalroller-cone bits provide the best SWD signals but may compromiseefficient drilling operations in many areas. Third, drilling withdownhole motors that rotate the bottomhole assembly while leaving therest of the drillpipe non-rotating has become a preferred method in manyareas, but that method also provides poor SWD signals. One methodproposed to improve SWD in these situations, such as disclosed by Barret. al. in U.S. Pat. No. 4,873,675 titled “Method and Apparatus forSeismic Exploration of Strata Surrounding a Borehole,” uses drillingjars, which are apparatus made to violently move the bottomhole assemblyup or down on demand to free stuck pipe, as the acoustic source ratherthan the drill bit. The drilling jar method involves downhole detectionof the reflected signal with a downhole geophone run on a cable with aside-door entry sub. Unfortunately, Barr's method is not feasible inmost situations because of the need for the cable, which is disruptiveto the drilling operation. Another method, disclosed by Beresford andCrowther in U.S. Pat. No. 5,798,488 titled “Acoustic Sensor,” uses adownhole acoustic transducer to both send and receive the acousticsignal. Beresford and Crowther do not disclose a method for determiningformation properties however.

[0009] Seismic data is also used to guide the drilling process, forexample to aid identification of potential high-pressure zones. However,seismic signal velocities are poorly correlated with high-pressurezones, and seismic data resolution is far below that needed to makedecisions during drilling. Increased seismic data resolution can beachieved by employing Vertical Seismic Profiling (VSP). In VSP,geophones are lowered into the borehole so that the precise depth of thegeophone is known and only the one way seismic travel times need to bemeasured. A major disadvantage of VSP, however, is that the drill stringmust be removed for VSP measurements. VSP data is therefore by necessityonly taken over limited intervals.

[0010] A method and apparatus is desired which will facilitate accurateestimation of the pore pressure in rock formations before suchformations have been penetrated by a drilling assembly. Preferably, thismethod and apparatus should not require withdrawal of the entire drillstring from the borehole each time measurement data is to be acquired,and should preferably allow generally continuous, if so desired,estimation of pore pressures in the formations directly ahead of thedrilling assembly. The present invention addresses these objectives.

SUMMARY OF THE INVENTION

[0011] The present invention involves use of a bottom hole assemblydeployed in a borehole to estimate formation properties. In theinvention a source signal is emitted from the bottom hole assembly andat least one signal is received by one or more receivers in the bottomhole assembly. Analysis of the frequency dependent characteristics ofthe received signal allows the estimation of the formation properties ofinterest.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The features of the present invention will become more apparentfrom the following description in which reference is made to thedrawings appended hereto. Reference numbers that are used in more thanone of the drawings refer to the same function in each drawing.

[0013]FIG. 1 shows a schematic diagram of a bottom hole assembly in aborehole and the signals that are generated and detected according tothe present invention.

[0014]FIG. 2 shows a schematic diagram of the relationship between porepressure, signal attenuation, and signal frequency.

[0015]FIG. 3 shows a form of the relationship between pore pressure,signal attenuation, and signal frequency from which a direct estimate offormation pore pressure can be made according to the method of thepresent invention.

[0016]FIG. 4 shows measurement data demonstrating the relationship ofcompressional wave velocity, shear wave velocity, and Poisson's ratio toeffective stress.

[0017] Changes and modifications in the specifically describedembodiments can be carried out without departing from the scope of theinvention, which is intended to be limited only by the scope of theappended claims.

DESCRIPTION OF THE INVENTION

[0018] The present invention is a system and method to estimateformation properties and in particular to estimate pore pressure in thevicinity of or in the formation ahead of the drill bit by analyzingacoustic waves that are emitted from the bottom hole assembly (BHA) andwhich pass through and are reflected from the formation. The acousticwaves used in the present invention are in the frequency range up toabout 20 kHz and can be generated passively, such as by the drill bit inthe drilling process, or actively, by placing a controlled acousticsignal source in the BHA. When active frequency sources are used, a muchwider range of frequencies may be employed, up to or greater than 100kHz. In the present invention acoustic detectors are mounted along theBHA to detect both compressional and shear waves. The pore pressureestimate for the formation ahead of the drill bit can be derived fromthe analysis of the frequency dependence of both the compressional waveamplitudes in the reflected signals and the change in velocities of thereceived signals. In addition, pore pressure may be estimated from thechange in the ratio of the compressional to the shear wave velocities inthe received signals. Other formation property estimates may also bederived from the analysis of the acoustic waves, such as fluidproperties and permeability.

[0019] For implementation in a measurement while drilling system, thepresent invention is a bottom hole assembly (BHA) which includes asource for generating the acoustic waves, a receiver array mounted onthe BHA for detecting the acoustic wave signals, a means for processingthe signals received by the receivers in a manner which allowsestimation of pore pressure information, and a means for transmittingsaid pore pressure information to the drilling rig on the surface. In analternate embodiment of the present invention, the received signals maybe transmitted to the surface, where the processing to estimate porepressure may be carried out.

[0020] The source of the acoustic signals may be the drill bit itself(in other words a passive source) or an active source mounted on theBHA.

[0021] For implementation in a measurement during well logging system,the present invention is a logging tool which includes a source forgenerating acoustic waves, a receiver array mounted on the logging toolfor detecting the acoustic signals, a means for transmitting thedetected signals to the surface control station, and a means forprocessing the detected signals at the surface to estimate the porepressure. Alternatively, the logging tool may be designed for processingthe acoustic signals on board the logging tool, with the processed data,which includes but is not limited to the pore pressure, then transmittedto the surface.

[0022] For convenience in reference herein but not to be limiting, theterm bottom hole assembly (BHA) will be used to refer to the downholeapparatus of the present invention, whether the implementation of thepresent invention is a measurement while drilling or a measurement whilewell logging system.

[0023] The present invention also includes a method to estimate the porepressure in the vicinity of the BHA. The method includes the steps ofdetecting actively or passively generated acoustic signals of varyingfrequencies, directly and after they are reflected from formations inthe vicinity of the BHA, determining frequency dependent properties,such as attenuation or velocity, of the detected signals, and estimatingthe pore pressure of formations in the vicinity of and ahead of the BHA.If so desired, other properties of formations in the vicinity of andahead of the BHA may also be estimated.

[0024] A first embodiment of the present invention is depicted inFIG. 1. The measurement system of this embodiment includes a source forgenerating acoustic signals at or near the drill bit; a receiver arraymounted on the BHA for detecting the signals generated by the source;hardware and software for signal processing, and a telemetry system fortransmitting data to the drill rig.

[0025] The source for generating acoustic signals may be either passiveor active. A passive source system will rely on the noise spectrum ofthe drill bit, which will typically involve large amplitude signals atfrequencies up to about 20 kHz, to generate the source signal. The drillbit noise spectrum will generally have its highest amplitudes in thefrequency of about 4 to about 6 kHz, and those amplitudes are typicallyfar larger than the amplitudes of signals in the 10 to 100 Hz rangewhich can be detected at the surface using seismic while drillingmeasurement techniques. The present invention teaches how to use thesehigher amplitude signals to image the formation rocks ahead of the drillbit through the use of suitable signal detectors, also referred to asreceivers, and signal processing components. More specifically, oneembodiment of the present invention involves the recording of theportion of the noise signal which is generated by and propagates aheadof the drill bit and reflects back to the BHA, and, using thecorrelation methods described further below, allows the estimation ofthe characteristics of the formation ahead of the bit.

[0026] An active source mounted in the BHA may also be used in thepresent invention. One advantage of using an active source is that dataprocessing will generally be simplified. A second advantage is that theenergy from the active source may be directed in specific directionsahead of the bit to increase the signal strength from the desiredreflectors. A variety of signal generation and data analysis options arepossible for active source embodiments of the present invention. Oneoption is to generate continuous signals with frequencies sweptrepetitively over a desired frequency range for a specified time period.Alternatively, a discrete set of frequencies over the desired frequencyrange can be generated. A pulsed excitation source may also be usedwhere the pulse width and amplitude are chosen so that the Fouriercomponents of the frequencies in the source signal span a desiredfrequency range. When using an active source, either continuous wave orpulsed, the source can be triggered during a hiatus in drilling, whichfurther simplifies data processing by eliminating the drill bit noisespectrum from the detected signals. Finally, if so desired, activesources frequencies up to or greater than 100 kHz may be acquired if sodesired.

[0027] Whether a passive or an active source is used in the presentinvention, bed boundaries, heterogeneities and other rock propertiescause changes in acoustic impedance which reflect some of the sourcesignal energy back to the BHA and which the receivers will detect. Thedepth of penetration of the source signals from the source to thereflectors will vary from several tens of feet when operating in arelatively high frequency range, from about 5000 Hz. and above, toseveral hundreds of feet when operating in a relatively low frequencyrange, from about 50 Hz. to 5000 Hz. If the drill bit noise spectrum isused as the source, the measured spectrum can be directlycross-correlated with the reflected signal to determine the time originand the distance to the reflector. If an active source is used,techniques similar to those used in reflection seismology orground-penetrating radar can be used to determine the distance to thereflectors.

[0028] The receivers must be mounted on the BHA since the signalfrequencies preferably to be used in the present method will notpropagate to the surface with measurable amplitudes. Receivers used inthe present method must be capable of measuring frequencies in the rangegenerated by the source. For a passive source system in which the drillbit noise spectrum is the signal, the receivers should preferably beable to measure signals up to about 20 kHz. If an active source is used,the receivers must be chosen to be able to measure the signalfrequencies that the source will generate. Preferably, receivers used inthe present method should be capable of measuring both compressionalwaves and shear waves. Note also that transducers which are capable ofboth generating and receiving acoustic signals may be used in thepresent invention.

[0029] In the embodiment of FIG. 1, several different signals will bedetected by the receivers. Bottom hole assembly 12, which extends intoborehole 6, is deployed, in a measurement while drilling systemembodiment, on the end of drill string 8. Note that the embodiment ofbottom hole assembly 12 in FIG. 1 is schematic in nature only and is notintended to be limiting. Bottom hole assembly 12 comprises a centermember 10, receivers 16, and, if data processing is performed downhole,will also include data processing components (not depicted in FIG. 1).Bottom hole assembly 12 as referred to herein means all components ofthe downhole apparatus below drill string 8 but above drill bit 14.Source signal 20 is emitted from a passive source, such as drill bit 14,or an active source (not depicted) in bottom hole assembly 12, andpropagates through first formation 3 to reflector 4, which is theboundary between first formation 3 and second formation 5. Receivers16A, 16B, 16C, and 16D may detect a number of different types ofsignals; FIG. 1 depicts examples of four of those types of signals.These signals, which will be discussed further below, will be generallyreferred to as receiver signals for convenience but not to be limiting.Note also that, in the following discussion the term signal will refergenerally to the wavefronts of the signals depicted in FIG. 1 andfurther discussed below. However FIG. 1 does not differentiate betweenthe compressional wave signal and the shear wave signal. It will beunderstood to those skilled in the art that both compressional and shearwaves have the same direction of propagation, for example as depicted inFIG. 1, but have different directions of particle motion within thewavefront. It will be understood that in a preferred embodiment of thepresent invention receivers 16 can sense both compressional and shearwaves signals. It will be understood that the present invention is notlimited to a bottom hole assembly which has four receivers 16, but thatthe four receivers 16 in FIG. 1 are depicted for convenience in thedescription of the present invention.

[0030] A first receiver signal which will be detected is the directarrival signal 22, which travels to receivers 16 along central member 10of bottom hole assembly 12. If source signal 20 is derived from apassive source, such as drill bit 14, the measurement of direct arrivalsignal 22 from drill bit 14 to receivers 16 serves to establish the timeorigin of source signal 20 which is required for the cross-correlationanalysis to be discussed below. This time origin determination is madepossible from calibration of the frequency dependent travel time alongthe central member 10 and the known distance from the passive source tothe receivers. To most accurately measure direct arrival 22, one of thereceivers 16, for example receiver 16A, should preferably be isolatedfrom source signal 20 and the other signals to be discussed furtherbelow. Isolating one of the receivers 16 from the rest of the othersignals is a hardware implementation issue that will be understood tothose skilled in the art.

[0031] In a measurement while well logging implementation of the presentinvention, an active source will be used and the time origin of thesource signal will be known from the time at which the source isactivated to generate a source signal. Therefore, it will generally beunnecessary to include an isolated receiver 16 in a measurement whilewell logging implementation of the present invention.

[0032] A second receiver signal detected by receivers 16 is known as thetube wave, or Stoneley wave, and is shown as direct borehole signal 24in FIG. 1. For clarity, direct borehole signal 24 is depicted with shortdashed lines in FIG. 1. Direct borehole signal 24 propagates from thesource within borehole 6 to receivers 16. Direct borehole signal 24 canbe used to estimate formation petrophysical properties, as is well knownin the art. More specifically, it will be understood that permeabilityof the formation surrounding the borehole (first formation 3 in FIG. 1extends upward from boundary 4 and surrounds borehole 6) is a propertywhich can be estimated from analysis of direct borehole signal 24.

[0033] A third receiver signal which is detected by receivers 16 isdirect formation signal 26 in FIG. 1, which travels directly from thesource to receivers 16 through the portion of first formation 3surrounding borehole 6. For clarity, direct formation signal 26 isdepicted with long dashed lines in FIG. 1. As will understood in theart, direct borehole signal 26 may be identified by linear move outanalysis of the data recorded by receivers 16, and may be used todetermine the frequency dependent velocity of first formation 3. Thefrequency dependent velocity may be used to determine the distance infirst formation 3 from the source, drill bit 14 in FIG. 1, to reflector4. Direct formation signal 26 may also be used to estimate petrophysicalproperties of first formation 3 according to embodiments of the methodof the present invention further described below.

[0034] A fourth receiver signal which is detected by receivers 16 isreflected signal 28, which is reflected from boundary 4 back towardsbottom hole assembly 12. Reflected signal 28 is a signal from whichchanges in formation properties in the region ahead of bottom holeassembly 12 may be determined. Because receivers 16 are positioned atvarying distances from the source, the establishment of the arrival timeof reflected signal 28 at each receiver allows the distance from source14 to boundary 4 to be determined using normal moveout methods. As willbe understood those skilled in the art, normal moveout methods are alsoused in surface seismic surveys. Reflected signal 28 will be identifiedby hyperbolic move out, and will provide the frequency dependentinformation from which embodiments of the method of the presentinvention allow the estimation of the pore pressure in second formation5.

[0035] When a passive source, such as drill bit 14, is used to generatesource signal 20, a cross-correlation analysis is used to identify thearrival times for all signals arriving at receivers 16. This processwill be well understood to those skilled in the art. First, a window intime is used to record the amplitude versus time characteristics of thesource noise spectrum that is present in direct arrival signal 22. Thelength of the time window is selected such that a truncated times seriesrepresenting source signal 20 is available which is independent of theother signals arriving at the receivers, and must be chosen to be ofsufficient width such that the desired frequency components of sourcesignal 20 can be identified. Those frequency components will beidentified in a Fourier transform of the truncated time series. Thearrival times for the other signals recorded by the receivers, in otherwords for direct borehole signal 24, direct formation signal 26, andreflected signal 28, are identified by individually cross-correlatingeach such signal with the source signal 20 which is determined from thistime window.

[0036] As will be understood to those skilled in the art thiscross-correlation involves the sliding of the time windowed sourcesignal to later time intervals which correspond to later arrival timesand convolving the source signal with the signals in these laterintervals. This frequency dependent cross correlation can be achieved byusing a notch filter and back transforming the Fourier transformedsource signal. Different notches can be used to selectively determinethe amplitudes for the desired frequency components.

[0037] When active sources are used, a frequency component analysis,such as a Fourier analysis or wavelet analysis, can be performed on thereceived signals to determine the frequency dependent velocity orattenuation. This simplification results from the known characteristicsof the active source signal. Such analyses are well understood to thoseskilled in the art.

[0038] One embodiment of a method of the present invention which allowsestimation of the pore pressure of second formation 5 at boundary 4 isfrom analysis of the frequency dependent amplitudes in reflected signal28. FIG. 2 depicts a representation of the relationship between porepressure (which is abbreviated Pp in FIG. 1; low pore pressure isrepresented by 54, high pore pressure is represented by 56), signalattenuation 52, and signal frequency 50 and is a simplification ofdetailed data in the published literature. In particular, the data inthe literature generally uses a horizontal axis which is the logarithmof the product of frequency and viscosity. FIG. 2 is simplified forconvenience, but not to be limiting. See for example O'Hara, Stephen G.,1985, “Influence of Pressure, Temperature, and Pore Fluid on theFrequency-Dependent Attenuation of Elastic Waves in Berea Sandstone,”Physical Review A, Vol. 32, No. 1, pp. 472-488, and O'Hara, Stephen, G.,1989, “Elastic-Wave Attenuation in Fluid-Saturated Berea Sandstone,”Geophysics, Vol. 54, No. 6, pp. 785-788. Note that the attenuation 52 ofthe amplitude of an acoustic signal (the vertical axis in FIG. 2) isessentially independent of frequency 50 at low pore pressure 54, whereasattenuation 52 increases with increasing frequency 50 at high porepressure 56. (Although the simplified depiction in FIG. 2 does not showunits for attenuation 52, it will be understood that acoustic signalattenuation is typically computed as the logarithmic decrement of asignal.) Thus, an analysis of the frequency dependence of theattenuation of the amplitudes of the signals received according to thepresent invention will reveal changes information pore pressure ahead ofthe bottom hole assembly.

[0039] The waveform processing to facilitate this analysis, includingcross correlation, data inversion to convert travel time to distance,and frequency component analysis, may be performed either downhole, bymicroprocessors or discrete logic components mounted in bottom holeassembly 12, or at the surface facility. If the processing is performeddownhole, the final pore pressure results are transmitted to the surfacedrilling rig by, for example, mud telemetry or other communicationmethods. If the processing is performed at the surface facility, thevarious measured signals are transmitted to the surface for analysis.

[0040] The results from which the simplification of FIG. 2 was derivedcan be further simplified as indicated in FIG. 3. In FIG. 3, a linearrelationship is shown to exist between pore pressure 58, depicted on thevertical axis and measured in units of ppg (pounds per gallon), and thelog of the slope of the lines derived by O'Hara in the data from whichFIG. 2 was derived, depicted on the horizontal axis and represented by60. Note that in the region of formation pore pressures 58 of interestto most drilling operations, 10 ppg to 20 ppg, the relationship islinear.

[0041] In this embodiment of the method of the present invention, theestimation of pore pressure follows directly from the data that ispresented in simplified format in FIG. 2, and the data in FIG. 3. Forestimating the pore pressure of a second formation 5 generally ahead ofthe BHA, a series of reflected signals 28 are recorded. These reflectedsignals 28 are analyzed to determine amplitude attenuation as a functionof frequency. Next, unless measurement or other data provides a specificviscosity value for use in the present embodiment, a fluid formationviscosity value is assumed. For formations in which gas may be present,a conservative assumption of gas viscosity may be used. For formationsin which shales are anticipated to be present, the viscosity of watermay be assumed, since, as will be understood to those skilled in theart, shale formations are generally water saturated. The basis forconservative assumptions of viscosity of fluids in other formations canbe determined by those skilled in the art. Next, a data curve for thesubject formation is plotted which has the measured frequency dependentamplitude attenuation values plotted (vertical axis) as a function ofthe logarithm of the product of frequency and viscosity (horizontalaxis). This data curve will reproduce for the subject data the plots ofO'Hara referenced above, and simplified in FIG. 2. Next, the logarithmof the slope of this data curve is computed, and the data of FIG. 3 isused to estimate the pore pressure 58 for the subject formation. Forreference, a three point linear regression fit of the data in FIG. 3above 10 ppg can be used to estimate pore pressure 58 according to thepresent embodiment. The data fit gives the following result:

Pore Pressure(ppg)=7.61+10.73*Log(Slope)

[0042] where Log(Slope) is the logarithm of the slope of the data curveof the subject formation. It will be understood in the art that theconfidence level associated with the pore pressure 58 which isdetermined from FIG. 3 will be a function of the signal-to-noise ratioof the measured signals, and that by increasing the number of signalmeasurements which are used to determine the data curve both thesignal-to-noise ratio and the pore pressure confidence level will beimproved. It will also be understood that the O'Hara data, which ispresented in simplified format in FIG. 2, from which FIG. 3 was derivedrelates to measurements made in Berea sandstone. The calculations ofpore pressure according to the present embodiment preferably will bebased on data that corresponds to the nature of the rock in theformation in which the bottom hole assembly is deployed. Analogous datafor other formation types can be obtained by persons skilled in the artfrom the published literature or from laboratory measurements.

[0043] A second embodiment of the method of the present invention allowsestimation of pore pressure from the frequency-dependent change invelocity of the signals that propagate back to bottom hole assembly 12.Several mechanisms have been proposed to account for the frequencydependent wave propagation properties of fluid filled porous rocks,including the Biot slow wave mechanism and the squirt flow mechanism. Ineither case, both a frequency dependent velocity as well as a frequencydependent attenuation will result, and both will vary with the porepressure. Thus, an alternate approach for estimating pore pressure aheadof the BHA is to measure the velocity of the waves traveling through theformation and reflected back to the receivers on the bottom holeassembly as a function of frequency. Following practices which areunderstood in the geophysical industry, wave propagation velocities as afunction of frequency can be determined from the time of arrival of thewave front at the receiver and the empirical velocity-to-pore pressurerelationships discussed above can then be used to estimate the porepressure of the formation ahead of the BHA.

[0044] A third embodiment of the method of the present invention allowsthe estimation of pore pressure from the calculation of the ratio of themeasured compressional wave velocity (υ_(p)) to the shear wave velocity(υ_(s)). Measured ultrasonic frequency data suggests that the ratioυ_(p)/υ_(s) increases by approximately 10% as the pore pressureincreases from a negligible value up to the confining pressure. See forexample, Christensen and Wang, 1985, “The Influence of Pore Pressure andConfining Pressure on Dynamic Elastic Properties of Berea Sandstone,”Geophysics, vol. 50, No.2, pp. 207-213. The Christensen and Wang datarelate changes in the conning and pore pressures in a formation to thePoisson's ratio. It will be understood to those skilled in the art thatPoisson's ratio can be directly calculated from the ratio of thecompressional wave velocity to the shear wave velocity. Thus, in thisembodiment of the method of the present invention, pore pressure may beestimated by analysis of the compressional and shear velocities of thereceived signals described above in conjunction with the Poisson's ratiorelationship to pore pressure data such as provided by Christensen andWang for Berea sandstone.

[0045] It will be understood to those skilled in the art that use ofthis embodiment of the method of the present invention to estimate porepressure from the ratio of compressional wave velocity to the shear wavevelocity requires data, such as that provided by Christensen and Wangfor Berea sandstone, which corresponds generally to the nature of therocks in the formation in which the bottom hole assembly is deployed.Analogous data for other rock types are available in the literature, forexample see Hamilton, E. L., “V_(p)/V_(s) and Poisson's Ratios in MarineSediments and Rocks,” J. Acoustic Soc. America, V. 66, No. 4, October1979, pgs 1093-1101. In addition, FIG. 4 shows data allowing thisembodiment to be used for formations comprised of Labette Shale. In thisplot the horizontal axis, effective stress 70, is the difference betweenthe mean confining stress and the pore pressure. The left vertical axisindicates both the compressional wave velocity 72 (diamond symbol) andthe shear wave velocity 74 (square symbol). The right vertical axisindicates the change in Poisson's Ratio 76 (triangle symbol) as thecompressional and shear wave velocities change. Since receivers 16 maybe used according to the present embodiment to measure both thecompressional wave component of reflected signal 28 and the shear wavecomponent of reflected signal 28, this data allows calculation of thevelocity ratio and the estimation of pore pressure Information onformation properties other than pore pressure may also be obtained withthe invention disclosed herein. For example, lithology and fluid contentare often estimated from compressional and shear wave signals. Theseestimates can be made from the signals that are detected and processedaccording to the present invention, thereby allowing estimation of theseproperties for the formation adjacent to and ahead of the BHA. Inaddition, it will be understood that compressional wave velocities maybe used to estimate rock strength. Other formation properties that maybe determined from the present invention will be known to those skilledin the art.

[0046] It will be understood that the present invention is not limitedmerely to sensing reflected signals from a single reflector, such asreflector 4 in FIG. 1. Rather, as will be understood to those skilled inthe art, additional reflectors will generally underlie the firstreflector, and signals will reflect from each such additional reflectorand be sensed by receivers 16. The present invention may be used toestimate the pore pressure and other formation properties of each suchadditional reflector.

[0047] As noted above, it will be understood in the art that theconfidence level associated with the formation property estimatesderived from embodiments of the method of the present invention are afunction of a number of factors, such as the signal-to-noise ratio ofthe measured signals and the extent to which the data derived from theliterature provide an accurate representation of the correlation ofvelocity to pore pressure for the subject formation or of thecorrelation of Poisson's ratio changes to pore pressure. In addition,persons skilled in the art will recognize that certain of the data inthe literature derive from measurements made at frequencies higher thanthe frequencies which are preferably employed in the method of thepresent invention. Such persons of skill in the art will thereforerecognize that increasingly accurate estimates of pore pressure andother formation properties of interest will be generated by ensuringthat any such datasets used in the present invention correspond to theexpected characteristics of the formations surrounding and ahead of theBHA 12 of the present invention.

[0048] It should be understood that the preceding is merely a detaileddescription of specific embodiments of this invention. Other embodimentsmay be employed and numerous changes to the disclosed embodiments may bemade in accordance with the disclosure herein without departing from thespirit or scope of the present invention. The preceding description,therefore, is not meant to limit the scope of the invention. Rather, thescope of the invention is to be determined only by the appended claimsand their equivalents.

We claim:
 1. A method of using a bottom hole assembly deployed in aborehole to estimate a formation property comprising the steps of: (a)generating a source signal from said bottom hole assembly; (b) detectingat least one receiver signal using said bottom hole assembly; (c)computing a frequency dependent characteristic of said at least onereceiver signal; and (d) using said frequency dependent characteristicto estimate said formation property.
 2. The method of claim 2 whereinsaid tool is a bottom hole assembly of a drilling apparatus.
 3. Themethod of claim 2 wherein said source signal is a noise spectrumgenerated by a drill bit of said drilling apparatus.
 4. The method ofclaim 3 wherein said step of determining frequency dependence is carriedout by cross-correlation analysis.
 5. The method of claim 4 wherein saidat least one receiver signal comprises a direct formation signal, andwherein said formation surrounds said borehole.
 6. The method of claim 4wherein said at least one receiver signal comprises a reflected signal,and wherein said formation is ahead of said borehole.
 7. The method ofclaim 1 wherein said frequency dependent characteristic is amplitudeattenuation.
 8. The method of claim 7 wherein the formation property ispore pressure.
 9. The method of claim 8 wherein said pore pressure isestimated from a frequency dependent attenuation relationship.
 10. Themethod of claim 1 wherein said frequency dependent characteristic iswave propagation velocity.
 11. The method of claim 10 wherein saidformation property is pore pressure.
 12. The method of claim 1 whereinsaid formation property is lithology.
 13. The method of claim 1 whereinsaid formation property is fluid content.
 14. The method of claim 1wherein said formation property is rock strength.
 15. The method ofclaim 1 wherein said tool is a bottom hole assembly of a measurementwhile well logging system.
 16. The method of claim 1 wherein said sourcesignal is generated by an active source located on said bottom holeassembly.
 17. The method of claim 16 wherein said step of determiningfrequency dependence is carried out by a frequency component analysis.18. The method of claim 1, wherein said at least one receiver signalcomprises a direct borehole signal.
 19. The method of claim 18 whereinsaid formation property is permeability.
 20. A method of continuouslyestimating the pore pressures of formations ahead of a bottom holeassembly, comprising the steps of a) generating a source signal fromsaid bottom hole assembly; b) detecting at least one receiver signalusing said bottom hole assembly; c) using said source signal and saidreceiver signal to estimate a pore pressure of at least one saidformation; and d) repeating steps a), b), and c) as said bottom holeassembly moves sequentially downward through said formations.
 21. Amethod of continuously monitoring the wellbore pressure safety margincorresponding to formations ahead of a bottom hole assembly, comprisingthe steps of a) generating a source signal from said bottom holeassembly; b) detecting at least one receiver signal using said bottomhole assembly; c) using said source signal and said receiver signal todetermine a pore pressure of said formation; d) using said pore pressureto monitor said wellbore pressure safety margin; and e) repeating stepsa), b), c) and d) as said bottom hole assembly moves sequentiallydownward through said formations.
 22. A method of continuouslyoptimizing the weight of drilling mud used in a drilling operation,comprising the steps of a) generating a source signal from a bottom holeassembly; b) detecting at least one receiver signal using said bottomhole assembly; c) using said source signal and said receiver signal todetermine a pore pressure of a formation ahead of said bottom holeassembly; and d) using said pore pressure to specify a weight of saiddrilling mud which corresponds to a target wellbore pressure safetymargin.